1. Technical Field
The present invention relates generally to a combustion monitoring apparatus for internal combustion engines, and more particularly to a combustion monitoring apparatus designed to distinguish abnormal combustion from normal combustion using the combustion ion current flow resulting from motion of ions within a combustion chamber produced by combustion of an internal combustion engine.
2. Background of Related Art
In modern gasoline engines for automotive vehicles, the optimum combustion is accomplished by monitoring a combustion condition within a combustion chamber of the engine to adjust the ignition timing or air-fuel ratio under feedback control. As such feedback control, a knock control system is well known in the art which has a knocking sensor disposed on a cylinder block of the engine to detect abnormal pressure vibrations in a combustion chamber during knocking. When the combustion knock has occurred, the ignition timing is retarded to decrease a combustion temperature to prevent the combustion knock from being increased. The preignition may also damage a spark plug or a piston. It is thus proposed to detect the preignition using a knocking sensor as used in the knock control system.
Usually, the preignition is caused by two factors: a temperature rise of a spark plug due to knocking and an increase in thermal insulation of a combustion chamber due to deposit on a chamber wall. The latter however cannot be sensed by the knocking sensor and may result in the so-called runaway preignition wherein the preignition compresses combustion gas to increase the temperature and pressure within an engine cylinder so that deposit on an exhaust valve becomes a heat source to heat the exhaust valve and the top of a piston further, thereby causing ignition to be advanced to the beginning of a combustion cycle, resulting in melting of the piston and valves ultimately.